Optical fiber amplifiers are used in a wide variety of important applications. An optical fiber amplifier typically includes a core region that is doped with at least one active element (e.g., a rare earth element) to provide gain. Examples of rare-earth dopants used in fiber amplifiers include Er, Yb, Nd, and Tu. Er-doped fiber amplifier technology is predominant in fiber optic communications applications because the range of wavelength over which Er-doped silica provides optical amplification (typically 1530 to 1580 nm) roughly coincides with the wavelength for minimum transmission loss in silica fiber. Yb-doped amplifiers, which typically provide amplification in the 1030 to 1100 nm range, are predominant in non-communications applications. This is primarily due to their high wall plug efficiency and scalability to extremely high average powers (multiple kW).
Roughly speaking, there are three distinct operating regimes for rare-earth-doped fiber laser technology. Fiber amplifiers can be configured as continuous wave (“cw”) sources, low-energy-pulse sources, and high-energy-pulse sources.
When operated in cw mode, the fiber laser output power as a function of time is nominally constant, and the population inversion of the gain medium is in steady state equilibrium. Energy is extracted from the gain medium by stimulated emission at substantially the same rate that energy is delivered to the gain medium by absorption of pump light photons. Common cw fiber laser configurations include the master oscillator power amplifier (“MOPA”), in which a fiber amplifier is seeded by a low-power cw seed source, and various cw fiber laser architectures in which some form of regenerative feedback (e.g. fiber Bragg grating mirrors) and a partially transmitting output coupler are used in conjunction with a fiber amplifier, either in a linear or ring-shaped cavity. Also included in this first category are “quasi-cw” fiber lasers, involving intermittent cw operation for periods of time well in excess of the energy storage time of the fiber amplifier gain medium.
When operated in the low-energy-pulse mode, the total energy extracted by each pulse is a very small fraction of that stored in the fiber amplifier gain medium. Therefore, as in a cw fiber laser, the population inversion as a function of time is substantially constant. Examples of fiber amplifiers operating in the low-pulse-energy regime include amplification of high-bit rate signals, and mode-locked fiber lasers. In the former application (e.g. telecommunications), the pulse train is a pseudo-random train of ones and zeroes that may be modeled as a square wave of very high frequency (e.g., GHz) having a nominal duty cycle of 50%. Mode-locked fiber lasers, on the other hand, generate a low duty cycle (e.g. 1%) periodic waveform in which the peak power of each pulse may exceed the average output power by two or more orders of magnitude. Nonetheless, the total energy extracted by each pulse is a very small fraction of the energy stored in the fiber amplifier gain medium, such that the population inversion of the gain medium is substantially constant as a function of time. Typical pulse energies, pulse durations, and pulse repetition rates for mode-locked fiber lasers may be 0.1 to 100 nJ, 0.1 to 1000 ps, and 1 to 100 MHz, respectively.
The high-energy-pulse regime is distinctly different than the cw and low-energy-pulse regimes in that amplification occurs under non-steady-state-equilibrium conditions. The energy extracted by each pulse is a significant fraction of that stored in the fiber gain medium, and the population inversion of the gain medium is not constant as a function of time. Fiber laser configurations for operation in the high-energy-pulse regime include q-switched fiber lasers, fiber amplifiers seeded by high-peak-power sources such as passively q-switched micro-chip lasers, and appropriately configured multistage fiber amplifier chains used in conjunction with a low-peak-power seed sources such as pulsed diode lasers. Typical pulse energies, pulse durations, and pulse repetition rates may be 10 to 1000 μJ, 1 to 100 ns, and 1 to 100 kHz, respectively.
The high-energy-pulse regime poses significant challenges to efficient operation of lasers and other types of amplifiers.